1. Field of the Invention
The present invention relates to a photosensitive resin composition comprising a photosensitive saponified poly(vinyl acetate), and to a pattern formation method making use of the composition. More particularly, the invention relates to a photosensitive resin composition which comprises a photosensitive saponified poly(vinyl acetate) dissolved in a high-boiling-point solvent having comparatively low toxicity, such as propylene glycol; which can be developed with water; has high photosensitivity; exhibits strong adhesion to a substrate; and has excellent patterning characteristics, and to a pattern formation method making use of the composition.
2. Background Art
Conventionally, a saponified poly(vinyl acetate) (hereinafter abbreviated as PVA)-base photosensitive resins having a pendant group derived from a styryl derivative having a quaternary aromatic azo-heterocyclic ring, which are disclosed in, for example, Japanese Patent Publication (kokoku) Nos. 56-5761, 56-5762, and 2-276806 and Japanese Patent Application Laid-Open (kokai) Nos. 56-11906, 59-17550, and 6-43645, are known to be water-soluble. These photosensitive resins are employed in practice as photosensitive materials for producing screen printing stencils, materials for making printing plates, materials for immobilization of biocatalysts, materials for manufacturing electronics, and other materials.
Quaternary styrylpyridinium or styrylquinolinium compounds are one of a rare type of photosensitive materials which undergo photo-dimerization via a singlet state, and advantageously, these compounds are not prone to inhibition by oxygen.
However, when a photosensitive resin having a styrylpyridinium or styrylquinolinium group or a derivative thereof is dissolved in an aqueous solvent and the solution is mixed with a material such as an inorganic material to thereby prepare a paste, the paste becomes dry when coated to a substrate through screen printing plate, thereby disrupting repeated printing. In addition, since film formed through such printing is easily dried, the paste exhibits poor smoothness and defoaming characteristics, thereby failing to provide a coating film of uniform thickness.
Furthermore, photosensitive resins having a styrylpyridinium or styrylquinolinium group or derivatives thereof can be dissolved in a highly polar solvent such as dimethylsulfoxide. However, these kinds of solvents are highly toxic, have bad odor and corrosive properties, and cannot be employed in some operation steps.
In contrast, PVA having a low saponification degree can be dissolved in certain organic solvents, but cannot be developed with water.
In view of the foregoing, an object of the present invention is to provide a photosensitive resin composition comprising a photosensitive saponified PVA which has a pendant group derived from a quaternary styrylpyridinium or styrylquinolinium compound or a derivative thereof and which can be dissolved in a relatively safe solvent of high boiling point such as propylene glycol, which composition can also be developed with water. Another object of the invention is to provide a pattern formation method making use of the composition.
The present inventors have conducted earnest studies in order to solve the aforementioned drawbacks, and have found that even when a hydrophobic group is introduced at a specific ratio into a photosensitive saponified PVA having a quaternary styrylpyridinium or styrylquinolinium moiety or a derivative thereof so as to enhance the hydrophobicity thereof such that the thus-modified resin or derivative can be dissolved in a high-boiling-point organic solvent such as propylene glycol, the resin or derivative can be developed with water without deterioration of properties such as excellent photosensitivity and adhesion. The present invention has been accomplished on the basis of this finding.
Accordingly, in a first aspect of the present invention, there is provided a photosensitive composition comprising a photosensitive saponified PVA, which composition comprises
a saponified PVA having the structural units represented by formulas (1) to (4): 
wherein each of k, l, m, and n is an arbitrary natural number; l may be 0; (k+l+m/2+n/2)=200-4500; R1 represents a hydrogen atom or a methyl group; R3 represents a hydrogen atom, an alkyl group, or an alkoxy group; R4 represents a hydrogen atom, a C1-C8 aliphatic hydrocarbon group, an aryl group, an aralkyl group, or a heterocyclic group; p is an integer of 1-6; q is 0 or 1; and R2 represents at least one species selected from the groups represented by formulas (8), (9), (10), and (11): 
wherein R9 represents a hydrogen atom, an alkyl group, or an alkoxy group; R10 is a member selected from the group consisting of a hydrogen atom, alkyl groups, aralkyl groups, and combinations thereof; wherein the alkyl group and the aralkyl group may or may not contain one or more members of the group consisting of a hydroxyl group, a carbamoyl group, an ether bond, an ester bond, and an unsaturated bond; R11 represents an alkylene group; Xxe2x88x92 represents a conjugate base; and Yxe2x88x92 represents SO3xe2x88x92 or CO2xe2x88x92;
and at least one organic solvent selected from the group consisting of compounds represented by formulas (5), (6), and (7): 
wherein each of R5 and R6 represents a hydrogen atom, a methyl group, an ethyl group, or an acetyl group; each of R7 and R8 represents a hydrogen atom, a methyl group, or an ethyl group; s is 1-4; and each of t and u is 1 or 2;
wherein the photosensitive saponified PVA is dissolved in the organic solvent.
Preferably, in formulas (1) to (4), (k+l+m/2+n/2) is 300-3,000, and the ratio of l to (k+l+m/2+n/2) is 50% or less.
Preferably, in formulas (1) to (4), the ratio of m to (k+l+m/2+n/2) is 0.5-5% and the ratio of n to (k+l+m/2+n/2) is 3-25%.
Preferably, in formula (4), R4 is a C2-C5 aliphatic hydrocarbon group.
The photosensitive composition comprising a photosensitive saponified PVA may further contain at least one of an inorganic powder and an organic powder, the powder being dispersed in the composition.
In a second aspect of the present invention, there is provided a pattern formation method which comprises applying to a substrate the photosensitive composition as recited in the first aspect; drying the formed layer; exposing the layer patternwise; and developing by use of a developer predominantly containing water.
Preferably, the photosensitive composition is applied to a substrate through screen printing or blade coating.